The present invention relates to a phase-change optical recording element that is particularly suitable for write-once read-many times (WORM) applications.
Optical recording has been increasingly used in recent years to publish, distribute, store, and retrieve digital information. This is done by focusing a laser beam to write and/or read information on an optical recording element usually in the form of a spinning disk. In the read-only memory (ROM) format, the information is prefabricated at the factory in the form of encoded small features on the element and the laser beam is used to read back the information. In the writeable formats, the laser beam is used to create small encoded marks through a variety of physical recording mechanisms. This permits the users to record their own data on the disk. Some recording physical mechanisms are reversible. The recorded marks can be erased and remade repeatedly. Disks that utilize these mechanisms are called erasable or rewriteable disks. Some of these physical mechanisms are one way, once the marks are made they cannot be reversed or altered without leaving a clearly identifiable trace that can be detected. Disks that utilize these mechanisms are called WORM (Write-Once-Read-Many times) disks. Each of these formats is suitable for certain practical applications.
The popularity of compact disk recordable (CD-R), which is a WORM disk, in recent years suggests the strong demand for WORM disks. WORM disks are suitable for many applications. In some of these applications, the data need to be stored in such a form that any modification to the content is not possible without leaving an easily detectable trace. For example, attempts to record over a previously recorded area may result in an increase in the read-back data jitter. An increase in data jitter of 50% is easily detectable and can be used to identify a recording element that has been modified. Recording elements that possess features that allow detection of modification attempts are hereto referred to as true-WORMs. In some other applications such publishing and data distribution, rewriteability is not necessary and the lower cost of WORM recording element makes them desirable. Yet, in some other applications some performance advantages of WORM recording elements, such as a higher writing speed, becoming the determining feature in choosing WORM over rewriteable elements.
Many physical mechanisms have been used for WORM recording. The first practical WORM optical recording element utilized ablative recording where the pulsed laser beam is used to create physical pits in the recording layer. This mechanism requires the recording elements to be in an air-sandwiched structure to leave the surface of the recording layer free from any physical obstruction during the pit formation process. This requirement not only increases the cost-but also introduces many undesirable properties that severely limit the usefulness of the recording element. Another mechanism is to use the laser beam to cause the fusing or chemical interaction of several layers into a different layer. This mechanism suffers from the requirement of relatively high laser power.
Yet another approach is to use organic dye as the recording layer. Although used successfully in CD-R disks, this mechanism suffers from its strong wavelength dependence. The optical head used in the DVD devices operating at 650 nm, for example, is not able to read the CD-R disks designed to work at the CD wavelength of 780 nm. Furthermore, a dye-based recording element tends to require more laser power for recording, and may have difficulties supporting recording at high speeds.
A more desirable approach is based on amorphous-crystalline phase-change mechanism. Phase-change material is the basis for the rewriteable DVD disks that have been introduced as DVD-RAM and DVD-RW products in the market. By properly selecting a different composition, the phase-change materials can be made WORM as well. A phase-change based DVD-WORM disk will have the best similarity in characteristics with the rewriteable DVD disks, and it can share the same manufacturing equipment with the re-writable disks. Both of these are highly desirable. Since the WORM feature requires disks that cannot be re-written, the phase-change materials for WORM needs to be different from those conventionally used for rewriteable disks. Commonly-assigned U.S. Pat. Nos. 4,904,577; 4,798,785; 4,812,386; 4,865,955; 4,960,680; 4,774,170; 4,795,695; 5,077,181 and 5,271,978, teach various alloys that can be used for write-once phase-change recording. When these alloys are used to construct a WORM optical recording element, the recording laser beam is used to change the atomic structure of the recording phase-change material from amorphous state to crystalline state. The unique feature that distinguishes these alloys from the conventional rewriteable phase-change materials is that the crystallization rate is so high at elevated temperatures just below the melting point, it is practically impossible to reverse the materials back into the amorphous phase once it is crystallized. Optical elements based on these alloys therefore possess true-WORM properties. Once the data are recorded on these elements, they cannot be altered without leaving a detectable trace. Optical recording elements based on these alloys, especially the ones using Sb100-m-nXmSnn based alloys, wherein, X is an element is selected from In, Ge, Al, Zn, Mn, Cd, Ga, Ti, Si, Te, Nb, Fe, Co, W, Mo, S, Ni, O, Se, Tl, As, P, Au, Pd, Pt, Hf, or V, as taught by commonly-assigned U.S. Pat. Nos. 4,904,577; 4,798,785; 4,812,386; 4,865,955; 4,960,680; 4,774,170; 4,795,695; 5,077,181 and 5,271,978, m and n represent the concentration of X and Sn in the alloy, have further advantages over other WORM optical recording elements. They are stable, having high recording sensitivity, and can be used in a simple, single-layer construction that drastically reduces manufacturing costs. However, recording elements based on these alloys also posses some shortcomings. One of the main shortcomings is the recent discovery that the recording performance of these elements deteriorates as the recording density is increased.
With the transition into the digital age, more and more digital data are generated everyday, and the need to store these ever increasing amounts of data keeps on increasing. There is therefore a strong need to keep increasing the density of the storage devices. In optical recording elements, this increase in density is achieved mainly through a decrease in the feature size used for storing information. To accomplish this decrease in feature size, the laser wavelength is being decreased and the numerical aperture of the focusing lenses is being increased to reduce the size of the read/write laser spots. However, the capability of the storage medium to support the small feature size is not guaranteed. In the ablative type media, frequently there is a rim around the ablative marks that physically prevents small features from being made. In the Sb100-m-nXmSnn phase-change alloys taught above, the noise increases when the recorded crystalline marks become smaller. The mechanism for this noise increase is not well understood. Transmission electron micrographs show the recorded marks in these alloys to generally consist of only a few crystalline grains, suggesting a low nucleation-site density in these alloy films. The low nucleation density has not presented a problem for lower density recording. When the recording density increases, however, the marks become smaller and the probability of proper nucleation during the irradiation time of the writing laser becomes smaller. Consequently, the recorded marks may become less uniform and the read back jitter increases. Adding oxygen (commonly-assigned U.S. Pat. No. 5,271,978), water, nitrogen, or methane (commonly-assigned U.S. Pat. Nos. 5,312,664 and 5,234,803) to these alloys improves the situation somewhat, but the small mark recording is still a problem.
Another shortcoming of the Sb100-m-nXmSnn alloy is the high optical density of the alloys. For certain applications, it is desirable to construct a multi-layer structure and utilize optical interference to enhance recording performance or to change the polarity of the recorded signals. For example, one can use a tri-layer structure comprising a phase-change recording layer, a dielectric layer, and a reflective layer; or a quadri-layer structure with an additional dielectric layer on the other side of the phase-change recording layer. For the optical interference to work, a substantial amount of light has to transmit through the phase-change layer and, therefore, the thickness of the phase-change layer has to be small. The required thickness decreases with increasing optical density of the phase-change layer. The Sb100-m-nXmSnn alloys have high optical absorption, with the imaginary part of the optical constant, k, larger than 3.0 in the amorphous phase and it increases to even higher values when the material crystallizes. When a Sb100-m-nXmSnn alloy thin film is used as the recording layer for a tri-layer or quadri-layer recording element, its thickness has to be so small that concern arises with respect to the film""s chemical stability. For operating at 650 nm wavelength, for example, the thickness of the phase-change recording layer needs to be less than 10 nm. The thickness of the dielectric layer also depends on the optical density of the phase-change layer: the thickness increases as the optical density increases. Since the deposition rates for dielectric layers are smaller than those for alloys, the need for a relative thick dielectric layer reduces the manufacturing throughput and increases product costs. The deposition process for dielectric layers are also hotter than that for alloys, long deposition time used for thick dielectric layers causes unwanted heating of the substrates. The high optical density of the Sb100-m-nXmSnn necessitates the use of thicker dielectric layer as well.
Furthermore, for some applications, it is necessary to use the differential-phase detection signal (DPD) for tracking. It has been found recently that tri-layer or quadri-layer recording elements using the Sb100-m-nXmSnn alloy do not have adequate DPD signal for reliable tracking.
It is therefore an object of the present invention to provide an improved, phase-change based WORM recording element that can support higher recording densities.
It is a further object of the present invention to provide an improved, phase-change material with lower optical densities to enable the construction of a WORM recording element in a tri-layer or quadri-layer structure that is more stable and easier to manufacturer.
It is yet another object of the present invention to provide an improved, phase-change based WORM recording element with improved differential-phase detection (DPD) signal.
These objects are achieved by using a WORM optical recording element comprising a substrate and a phase-change recording layer wherein the phase-change recording layer has a composition expressed by SbaXbSncZndSieOfSh wherein X is an element is selected from In, Ge, Al, Zn, Mn, Cd, Ga, Ti, Si, Te, Nb, Fe, Co, W, Mo, S, Ni, O, Se, Tl, As, P, Au, Pd, Pt, Hf, or V, and a greater than 0, b greater than 0, c greater than 0, d greater than 0, e greater than 0, f greater than 0, h greater than 0, and a+b+c+d+e+f+h=100. Most preferably, the phase-change layer has a composition expressed by SbaInbSncZndSieOfSh wherein a greater than 0, b  greater than 0, c  greater than 0, d greater than 0, e  greater than 0, f greater than 0, h greater than 0, and a+b+c+d+e+f+h=100.